A review of patents indicates calipers have been fitted with accessories to improve and to expand their use in checking the measurements and sizes of machined parts and products. In 1872, E. C. C. Kellogg in his U.S. Pat. No. 129,349 provided slide on tiltable arm accessories for the legs of calipers to measure tapered surfaces. In 1878, Anton Schmitter in his U.S. Pat. No. 207,308, provided slide on accessories for the legs of calipers to change the calipers from the inside to outside calipers without changing the position of the slide of the caliper. In 1928, A. Hilfiker in his U.S. Pat. No. 1,659,915 illustrated caliper attachments for measuring undercut beveled surfaces. In 1949, John Krisanda in his U.S. Pat. No. 2,467,263 disclosed removable caliper attachments to add to the measuring jaws, i.e. legs, or blades, of his calipers. They were divider and caliper point accessories for making additional measurements, not possible to be undertaken when only the original measuring jaws were available. In 1950, John Gallup in his U.S. Pat. No. 2,529,931 depicted his auxiliary selective contact accessories for calipers. The respective contact accessories were used to measure beveled surfaces, and other irregular surfaces such as internal and external gear teeth. In 1980, Thomas Schasteen in his U.S. Pat. No. 4,202,109, set forth his thread gauge, which is an attachment for the blades of a caliper to be used in measuring the pitch diameter of threads of a machined product such as a bolt. In 1981, Arley Heinayer in his U.S. Pat. No. 4,291,465, illustrated his caliper accessory for extending the measurement range of the caliper. These inventors disclosed how conventional calipers could be equipped with accessories to increase the types of measurements which could be undertaken.
Even though the prior offered measuring instruments and their accessories appear at first to have filled all the needs of a machinist, there still remained a need whereby a machinist could be provided with accessories to make his vernier and/or dial calipers more universally useful in determining very accurately the correctness of many difficult to reach places on products being machined. In so fulfilling the need, the machinist would be able to quickly mount, remove, and change the accessories without damaging his or her calipers or the accessories, and moreover, the machinst could imaginatively add to the configurations of the contacting points, blades, bars, pins, etc.